refactor(memory): use sync.Map for session locks and skip-scan in readMessages

Address review feedback from @Zhaoyikaiii:

- Replace map[string]*sync.Mutex + separate mu with sync.Map.LoadOrStore
  for simpler, lock-free session lock management.

- Add skip parameter to readMessages so callers (GetHistory, Compact)
  can skip truncated lines without paying the json.Unmarshal cost.

- Add countLines helper for TruncateHistory's count reconciliation,
  avoiding full deserialization when only the line count is needed.

pkg/memory/jsonl.go

@@ -36,10 +36,8 @@ type sessionMeta struct {
 // GetHistory ignores lines before that offset. This keeps all writes
 // append-only, which is both fast and crash-safe.
 type JSONLStore struct {
-	dir string
-
-	mu    sync.Mutex
-	locks map[string]*sync.Mutex
+	dir   string
+	locks sync.Map // map[string]*sync.Mutex, one per session
 }
 
 // NewJSONLStore creates a new JSONL-backed store rooted at dir.
@@ -48,23 +46,13 @@ func NewJSONLStore(dir string) (*JSONLStore, error) {
 	if err != nil {
 		return nil, fmt.Errorf("memory: create directory: %w", err)
 	}
-	return &JSONLStore{
-		dir:   dir,
-		locks: make(map[string]*sync.Mutex),
-	}, nil
+	return &JSONLStore{dir: dir}, nil
 }
 
 // sessionLock returns (or creates) a per-session mutex.
 func (s *JSONLStore) sessionLock(key string) *sync.Mutex {
-	s.mu.Lock()
-	defer s.mu.Unlock()
-
-	l, ok := s.locks[key]
-	if !ok {
-		l = &sync.Mutex{}
-		s.locks[key] = l
-	}
-	return l
+	v, _ := s.locks.LoadOrStore(key, &sync.Mutex{})
+	return v.(*sync.Mutex)
 }
 
 func (s *JSONLStore) jsonlPath(key string) string {
@@ -122,9 +110,11 @@ func (s *JSONLStore) writeMeta(key string, meta sessionMeta) error {
 	return nil
 }
 
-// readMessages reads all valid JSON lines from a .jsonl file.
+// readMessages reads valid JSON lines from a .jsonl file, skipping
+// the first `skip` lines without unmarshaling them. This avoids the
+// cost of json.Unmarshal on logically truncated messages.
 // Malformed trailing lines (e.g. from a crash) are silently skipped.
-func readMessages(path string) ([]providers.Message, error) {
+func readMessages(path string, skip int) ([]providers.Message, error) {
 	f, err := os.Open(path)
 	if os.IsNotExist(err) {
 		return []providers.Message{}, nil
@@ -139,11 +129,16 @@ func readMessages(path string) ([]providers.Message, error) {
 	// Allow up to 1 MB per line for messages with large content.
 	scanner.Buffer(make([]byte, 0, 64*1024), 1024*1024)
 
+	lineNum := 0
 	for scanner.Scan() {
 		line := scanner.Bytes()
 		if len(line) == 0 {
 			continue
 		}
+		lineNum++
+		if lineNum <= skip {
+			continue
+		}
 		var msg providers.Message
 		if json.Unmarshal(line, &msg) != nil {
 			// Corrupt line — likely a partial write from a crash.
@@ -162,6 +157,30 @@ func readMessages(path string) ([]providers.Message, error) {
 	return msgs, nil
 }
 
+// countLines counts the total number of non-empty lines in a .jsonl file.
+// Used by TruncateHistory to reconcile a stale meta.Count without
+// the overhead of unmarshaling every message.
+func countLines(path string) (int, error) {
+	f, err := os.Open(path)
+	if os.IsNotExist(err) {
+		return 0, nil
+	}
+	if err != nil {
+		return 0, fmt.Errorf("memory: open jsonl: %w", err)
+	}
+	defer f.Close()
+
+	n := 0
+	scanner := bufio.NewScanner(f)
+	scanner.Buffer(make([]byte, 0, 64*1024), 1024*1024)
+	for scanner.Scan() {
+		if len(scanner.Bytes()) > 0 {
+			n++
+		}
+	}
+	return n, scanner.Err()
+}
+
 func (s *JSONLStore) AddMessage(
 	_ context.Context, sessionKey, role, content string,
 ) error {
@@ -234,18 +253,13 @@ func (s *JSONLStore) GetHistory(
 		return nil, err
 	}
 
-	msgs, err := readMessages(s.jsonlPath(sessionKey))
+	// Pass meta.Skip so readMessages skips those lines without
+	// unmarshaling them — avoids wasted CPU on truncated messages.
+	msgs, err := readMessages(s.jsonlPath(sessionKey), meta.Skip)
 	if err != nil {
 		return nil, err
 	}
 
-	// Apply logical truncation: skip the first meta.Skip messages.
-	if meta.Skip > 0 && meta.Skip < len(msgs) {
-		msgs = msgs[meta.Skip:]
-	} else if meta.Skip >= len(msgs) {
-		msgs = []providers.Message{}
-	}
-
 	return msgs, nil
 }
 
@@ -299,11 +313,11 @@ func (s *JSONLStore) TruncateHistory(
 	// If the meta count might be stale (e.g. after a crash during
 	// addMsg), reconcile with the actual line count on disk.
 	if meta.Count == 0 {
-		msgs, readErr := readMessages(s.jsonlPath(sessionKey))
-		if readErr != nil {
-			return readErr
+		n, countErr := countLines(s.jsonlPath(sessionKey))
+		if countErr != nil {
+			return countErr
 		}
-		meta.Count = len(msgs)
+		meta.Count = n
 	}
 
 	if keepLast <= 0 {
@@ -369,17 +383,13 @@ func (s *JSONLStore) Compact(
 		return nil
 	}
 
-	all, err := readMessages(s.jsonlPath(sessionKey))
+	// Read only the active messages, skipping truncated lines
+	// without unmarshaling them.
+	active, err := readMessages(s.jsonlPath(sessionKey), meta.Skip)
 	if err != nil {
 		return err
 	}
 
-	// Keep only the active (non-skipped) messages.
-	var active []providers.Message
-	if meta.Skip < len(all) {
-		active = all[meta.Skip:]
-	}
-
 	err = s.rewriteJSONL(sessionKey, active)
 	if err != nil {
 		return err

pkg/memory/jsonl_test.go

@@ -440,7 +440,7 @@ func TestCompact_RemovesSkippedMessages(t *testing.T) {
 	}
 
 	// Before compact: file still has 10 lines.
-	allOnDisk, err := readMessages(store.jsonlPath("compact"))
+	allOnDisk, err := readMessages(store.jsonlPath("compact"), 0)
 	if err != nil {
 		t.Fatalf("readMessages: %v", err)
 	}
@@ -455,7 +455,7 @@ func TestCompact_RemovesSkippedMessages(t *testing.T) {
 	}
 
 	// After compact: file should have only 3 lines.
-	allOnDisk, err = readMessages(store.jsonlPath("compact"))
+	allOnDisk, err = readMessages(store.jsonlPath("compact"), 0)
 	if err != nil {
 		t.Fatalf("readMessages: %v", err)
 	}